2003 Piru Fire Historical Imagery

The increased occurrences of drought and fire may be contributing to the
loss of biodiverse ecosystems in Mediterranean regions. Specifically, the
conversion of diverse native shrublands, such as chaparral, to non-native
annual grassland by fire is of great conservation concern in California.
To avoid or slow the loss of chaparral, it is important to understand the
underlying causes of landscape conversion. Studies investigating the
interaction of multiple potential drivers are particularly crucial to
identification of vulnerable areas of the landscape. Here we used aerial
imagery to evaluate vegetation transitions between chaparral, sage scrub,
grassland, and tree domination and their potential drivers within Ventura
County, California, a strongly Mediterranean climate region. We used
random forest algorithms and conditional inference trees to determine the
climatic, topographic, and fire-related variables contributing most to
vegetation change. Our results support that chaparral conversion to grass
(27% of chaparral plots) is a result of landscape position, fire, and
drought acting in tandem. In particular, lower elevation, southwest facing
slopes that experience a post fire drought are at very high likelihood of
conversion to non-native annual grass. Additionally, our results show that
these grasslands, once formed, rarely convert to other community types.
Therefore, protecting shrub dominated areas that are most likely to
convert (low elevation, more southwest facing slopes, less annual
precipitation) is crucial to preserving native vegetation diversity.